For the last several decades, there have been increasingly stringent emission standards applied to internal combustion engines of all types. Concurrent with the move towards stricter limits on emissions was the adoption of particulate filters. Such adoption was first made in the field of compression ignition, or diesel, engines operating under such load and rpm conditions that soot is generated in the exhaust and is required to be filtered. With advances in the fuel efficiency for other types of engines, for example, spark ignition engines with direct fuel injection, the need for a particulate filter may also be indicated.
Whatever the type of engine, the particulate filter has evolved to a very high efficiency, trapping above 90% of soot with a wall flow filter. When sufficient soot has been deposited on the walls, the pressure drop increases across the particulate filter and an even higher soot trapping efficiency is achieved. It is common to measure pressure drop across a particulate filter through the use of a delta pressure sensor, used to predict soot loading. Typically, these predictions are made with models such as those disclosed by Konstandopoulos, et al. (SAE paper 2002-01-1015). The delta pressure reading is converted to a normalized pressure differential using equations set forth in the above referenced SAE paper and these are used to determine when the particulate filter trap needs to be regenerated in order to remove the soot in the trap.
The particulate filter, having a high efficiency, also traps ash, which comes from high ash lubricating oil, excessive oil consumption, and the use of high ash fuels, such as biodiesel. As ash gradually accumulates in the particulate filter, the delta pressure signal at a given soot load will be higher. This consequence is a result of ash occupying space in the inlet channels of the particulate filter, leaving less surface or volume for soot distribution providing an obstruction to gas flow that increases the pressure drop across the particulate filter.
Overall, ash accumulation is generally a slow process. Total exhaust system back pressure due to ash starts to become noticeable in the regeneration intervals generally above 2,500 hours of engine operation for engines having a power output of greater than 130 kilowatts (174 HP). For engines having lower than 130 kilowatts output, the ash effect can occur above 1,500 hours. In addition to the indication of more frequent regeneration of the particulate filter, the accumulation of ash affects the engine performance overall due to increasing back pressure. Without any compensation for ash loading, the time interval between regeneration starts to decrease, since the system typically determines whether regeneration should occur based on delta pressure. In addition to the ash loading having an effect on regeneration intervals, it also can affect the service life of the particulate filter, that is the point at which the filter needs to have accumulated ash removed therefrom.
What is needed in the art, therefore, is a method for reliably predicting the service life of a particulate filter in an internal combustion engine system.